1. Field of the Invention
The invention relates to a process, using regenerable adsorption materials, for purifying exhaust gases that have been contaminated with at least SO.sub.2, a heavy metal, in particular mercury, and other toxic gases, in particular dioxins and furans, and that originate from incineration plants, in particular waste-incineration plants.
2. Description of Related Art
Exhaust gases from waste incineration plants contain HCl, HF, SO.sub.x Hg and NO.sub.x, as well as highly toxic organic PCDD/PDCF compounds, known as dioxins and furans. In order to maintain clean air quality, the contaminants from the exhaust gases need to be eliminated and as far as possible reclaimed as re-usable materials.
Double or multi-stage lime-based wet-processes are known for the separation of HCl, HF and SO.sub.2, which produce mainly CaCl.sub.2 and CaSO.sub.4 as reaction products. These reaction products must be taken and stored in waste disposal sites. A further disadvantage of these processes is that they produce contaminated waste waters that must be either subjected to purification or thermally disposed of. Only after such waste waters have been purified can they be conducted into a receiving canal or into the sewage system. This whole process is expensive, and damaging to the environment, because of the storage of the reaction products in waste disposal sites. There are known designs (cf. Chem.-Ing. Tech. 60, 1988, Pages 247-255) by which the heavy metals extracted during the purification in the wet-cleaning process can in principal be reprocessed and recovered. Since these processes are very expensive, with an unfavorable cost-benefit ratio, underground disposal is regarded as economically sensible and ecologically permissible. In the recovery of heavy metals in a large power plant, 2000 tons of salts per year are produced, which must then be subjected to controlled release into the sea. As for furans and dioxins, these can be easily destroyed at moderate temperatures by recycling the gases contaminated with them back through the incinerator.
There are proposals known for using a caustic soda solution as a neutralization medium to recover NaCl as an industrially marketable product, and thus reduce the amount of disposable waste material. This procedure is expensive and would only be feasible, given the high purity requirements for NaCl used in chlorine-alkali-electrolysis, if the HCl contained in the exhaust gas had previously been thoroughly separated from all remaining contaminants, something which is not possible using the known processes for producing NaCl.
There are also dry sorption processes known, that operate dry or almost dry and are also lime-based. The resulting reaction products are disposed of, along with the flue-dust from the incineration, as waste. In this process a further problem arises, since the dioxins and furans contained in the exhaust gas are not eliminated.
In both the dry and wet processes there is the problem of separating the mercury, present in gaseous form, from the exhaust gas. In the dry sorption process this separation is impossible, and in the wet-wash process it is at best unreliable.
In DE 37 06 131 A1 a process is known for removing contaminants from the flue gas, according to which the gas is previously treated in a wet scrubber and passed through a series of adsorbent beds. This process takes advantage of the fact that contaminants accumulate in the adsorber with a specific selectivity, so that smaller molecules are expelled from the adsorbent when it reaches saturation. The gas treatment provides for the removal of such components from the exhaust gas, since otherwise they could impede a subsequent catalytic denitration. In a first adsorber layer heavy metals, in particular mercury, are adsorbed. In one or two further layers SO.sub.2 and HCl are adsorbed. Later, the contaminated adsorption material is passed through a further adsorption stage, in which excess ammonia from the denitration stage, contained in the gas, is adsorbed. The adsorption material, contaminated with ammonia, SO.sub.2 and HCl, is subsequently burned in the incinerator. The coke, contaminated with heavy metal from the first adsorption stage, is removed and disposed of. The most common adsorption material used is (nonregenerable) smelting coke. This process can only be used subsequent to a prepurification of the flue gas, since otherwise all the adsorbed contaminants would be introduced back into the flue gas by the burning of the adsorbent. It has also been suggested that expensive varieties of activated coke could be used in the adsorption stages for the gaseous wastes and then regenerated. Nothing is said about what would happen to the contaminants that would be released during regeneration.
For the reasons cited, the known process is only usable for the final purification of previously purified flue gas.
Incinerating the waste-contaminated smelting coke carries with it the risk that dioxins and furans will not remain in the incinerator long enough to be destroyed, or that PCDD/PCDF recombinations may form because of the presence of O.sub.2 and Cl.sub.2 in the incinerator area, which may even be promoted by the catalytic effects of copper particles in the waste or in the flue-dust, and that these may result, together with the undestroyed dioxins and furans, in a gradual build-up of these highly toxic contaminants in the exhaust gas. The only way to exclude the dioxins and furans from the exhaust gas purification system, would be to dispose of the poison-contaminated smelting coke, which would increase yet further the amount of waste products to be disposed of.
"Energie Spectrum" of July 1989, (pages 13-16) published an overview of features of conventional processes from various suppliers. From this it is clearly the case with all adsorption processes, either that they result in waste products that must be disposed of, or that the contaminated adsorption coke and coal is incinerated, which for the reasons mentioned earlier is only advisable if the adsorption is used solely for the final purification of flue gases.
With the known dry sorption processes, alkaline additives are introduced to the burning material, in order to reduce the discharge of acidic gases from the waste products. The treatment of these contaminant-reduced flue gases with adsorbers also leads to considerable problems. For example, mercury cannot be separated from the lime products used in the dry sorption process. Incinerating the contaminated smelting coke in the burner would lead to a gradual build-up of mercury in the exhaust gas. This has led to the idea of processing the smelting coke so that the residual SO.sub.2, HCl and HF adsorbed by the coke, as well as the mercury from the coke, is thermally desorbed; the mercury is then readsorbed onto sulphur-treated coke, and the remaining wastes together with the decontaminated coke are returned to the burner. The sulphur-treated and mercury-contaminated coke has to be disposed of in special storage.
Meanwhile, dry sorption has led in yet another direction, in order to isolate the dioxins and furans and the mercury. A small amount of smelting coke is added to the lime that is used in the dry adsorption to help separate out the SO.sub.2, HCl and HF, and this coke adsorbs the dioxins and furans as well as the mercury. In addition the smelting coke adsorbs certain further amounts of SO.sub.2, HCl and HF.
This process however only leads to an increase in waste products, since the contaminated coke, along with the products of the lime reaction, most often still mingled with flue dust, must be transported to disposal sites and stored there.
There is still no long-term experience with the storage of smelting coke that has been contaminated with dioxins and furans, mercury, SO.sub.2, HCl and HF. Even storage in special disposal sites seems questionable, in light of the dioxins and furans and mercury present. Using this process for purifying the exhaust gases from waste incineration plants would, besides, increase disposal costs considerably, because of the need for special storage facilities for reaction products that hitherto could be more cheaply stored in single-purpose disposal sites.
Furthermore, the application of this process makes it essential to use in addition an activated charcoal filter after the dry sorption for the separation of residual SO.sub.2, HCl and HF, since with dry sorption it is difficult to achieve the required low concentrations of contaminating residue in the purified gas. Incineration of the contaminated coke in the burner is acceptable in this case, however, since only SO.sub.2, HCl and HF are released, and the dry sorption serves as a sink for them. The activated charcoal filter serves here exclusively for the final purification of the exhaust gases, but not however for the separation of dioxins, furans and Hg.
All known flue-gas treatment processes give rise to waste products that must be disposed of, if the build-up of toxic substances in the exhaust gas is to be avoided.
A process is known, from DE 34 26 059 A1, specifically for removing organic contaminants, in particular dioxins and furans, from flue-gases through adsorption. This separation should at the same time make it possible to remove other contaminants like SO.sub.2 and heavy metals. The activated charcoal or coke used for the adsorption is subjected to the conventional regeneration process with inert gases in the temperature range of about 350.degree.-750.degree. C. To split out the dioxins and furans, the contaminated desorption gas drawn from the regeneration process is heated to a temperature of over 1,000.degree. C., up to about 1,400.degree. C. In this way the decomposition or splitting temperature for dioxins and furans should be exceeded, thus ensuring that they are destroyed. The required duration of this splitting temperature is in the order of a few seconds, perhaps 5-10 seconds. In this way it should be possible to destroy dioxins and furans in the course of desorption in the regeneration stage. The publication presents no design for the further treatment of the remaining contaminants.
The problem addressed by this invention lies in the context of avoiding the production of wastes that must be disposed of, or exhaust gases that harm the environment, and involves the description of a process for purifying exhaust gases of their content of SO.sub.2, and mercury and other toxic gases, in such a way as to allow the profitable recovery of materials.